Linear array ultrasonic scanning systems are utilized in a variety of applications including medical imaging. A particular application of such systems is to utilize the Doppler effect to determine the direction and, more importantly, the rate of flow of a fluid such as blood at a gate or sample volume point. Such a system may for example operate by applying bursts of pulses at a selected repetition rate from a selected subset or aperture of the ultrasonic transducers of the linear array to the sample volume of the subject being examined. By positioning the sample volume at a selected point or successive sequence of points along a vein or artery, it is thus possible to plot blood flow through such internal channel. Abnormalities in the rate of blood flow at a selected sample volume are indicative of various disease conditions such as a partial or total occlusion in such channel.
In order to take Doppler readings for a selected sample volume, it is necessary that the direction of the parallel scan lines from a selected aperture of the linear array transducers be at an angle to the direction of flow which is not 90.degree. or 270.degree. since it is not possible to obtain Doppler readings at these angles. For most applications, the ideal scan line angle is in a direction parallel to the direction of flow since this maximizes the Doppler difference. However, since extensive diagnostic tables exist for an assumed scan angle of 60.degree. (or 150.degree.) to the direction of blood flow, in some applications this may be the ideal scan direction angle rather than 0.degree. or 180.degree..
While an ideal scan angle may exist, it is not always possible to utilize this angle for a variety of reasons. First, because of the position of the sample volume within the viewing area, it may not be possible to set the transducers at the ideal angle while still passing through the sample volume. A second potential problem is that the ideal angle may be greater than the maximum steering angle available for the transducers of the linear array. For example, the system may be designed to provide steering angles of only plus or minus 45.degree. for the ultrasonic transducers. If the ideal scan angle for a given sample volume is greater than the available steering angle, an angle other than the ideal scan angle must be utilized for such sample volume.
Finally, the depth of the sample volume, which is defined as the distance from the center transducer of the transducer aperture for the scan to the sample volume, is one factor in determining the time required for a pulse to reach the sample volume and for an echo of such pulse to return to the transducer. This round trip time for a sampling pulse determines the maximum pulse repetition rate available for the scan. The pulse repetition rate in turn determines the maximum blood flow rate which may be detected by the system. Thus, where because of diseased arteries or other blood flow channels, a high blood flow rate may be anticipated, and thus a high pulse repetition rate from the Doppler linear array may be required, depth may become a limiting factor on the angle which can be utilized for a given scan, particularly if the blood channel of interest is not near the skin surface.
Thus, when doing Doppler measurements with an ultrasonic linear array system, it is desirable that it be possible to select a scan line for the scan of a given sample volume which is as close as possible to the ideal angle from the direction of blood flow at the sample volume, provided that the depth from the aperture being used on the transducer array to the sample volume at the selected angle does not exceed the depth which would permit a desired pulse repetition rate for the Doppler scan.
Heretofore, the selection of the scan direction for Doppler scanning has been done manually. In particular, the angle for the scanning beam has been fixed at, for example, 60.degree., and the system operator has been able to manually move the transducer aperture until an aperture is found which causes the beam to pass through the selected sample volume. The system operator thus could not control the angle to achieve a desired angle and, in the remote case where the fixed direction of the beam happened to be perpendicular to the direction of blood flow, the system could not be utilized. In most instances the angle utilized was not the ideal angle, or even near the ideal angle, for the particular sample volume. Further, if the depth at the fixed angle turned out to be greater than the depth required in order to achieve a desired pulse repetition rate, there was no easy way to compensate for this and a lower pulse repetition rate would have to be utilized. The only other option would be to reposition the transducer array on the patient and start over.
Thus, even though such systems provide only limited control over Doppler beam direction and a beam direction which could be far from optimum, adjusting the beam for each scan is still a relatively time consuming process. The time required to make the manual adjustments could become a problem where, as is frequently the case, a large number of sequential readings are taken along a blood channel.
A need therefore exists for a relatively simple and rapid method and apparatus for permitting the optimum available scan angle to be achieved for each Doppler measurement, with the optimum available angle being determined taking into account all factors including the maximum steering angle of the transducers, the position of the sample volume within the field of view of the transducers and the maximum depth available to achieve a desired pulse repetition rate.